A Neutron Reflection Study of the Dissolution of Miscible Glassy Polymer Films over a Range of Temperature

Although the mixing of miscible liquids is generally thought to be relatively well-described by classical Fickian diffusion models, it is well-known that the physics of glass-formation can greatly alter the dissolution dynamics of polymer materials brought into contact with solvents and other polymer materials. Despite the immense practical importance of this phenomenon in many contexts where polymers are "blended" (e.g., polymer recycling), models of the dissolution dynamics of glassy polymer materials tend to be highly phenomenological and to have limited general applicability. This situation is understandable given the limited fundamental understanding generally of glass formation, pointing to the need for high-resolution measurement methods on model materials and an appropriate theoretical framework to elucidate the fundamental nature of the polymer dissolution process under realistic physical conditions. In the present work, we restrict our consideration to the interfacial dynamics of a model system of this kind, composed of two thermodynamically miscible, but glassy, polymer films formed through spin-coating and film floating techniques. We utilize neutron reflectivity (NR) to characterize the interfacial mixing dynamics in these stacked glassy polymer films. The initial stage of the interfacial mixing is found to be somewhat similar to Fickian interdiffusion of miscible liquids, although we observe a fractional power-law growth of the interfacial width in this transient regime with an exponent near 1/3, as commonly observed in the coarsening of phase-separating polymer blends. After an induction time t* over which the interfacial concentration approaches that of the fully dispersed polymer mixture, we generally observe a transition to a regime in which the "mixed" polymer in the interfacial region between the films invades both films in a front-like fashion. We adapt phase field modeling of "solid" material dissolution to qualitatively understand and characterize this evidently complex non-Fickian dissolution process. The rates of the propagating composition wave, as well as the evolution of the interfacial width separating the relatively stable "uniformly mixed" polymer material from the relatively unstable unmixed polymer material, are quantified. Interestingly, this frontal dissolution process in glassy miscible polymer films closely resembles the phenomenology of Case II non-Fickian diffusion commonly observed in the dissolution of glassy polymers by solvents, and we suggest that the latter corresponds to the situation in which one of the layer components is not glassy so that the dissolution wave propagates only in one direction.